Thank you for your excellent question.
If your uncle was competent and you did not unduly influence him into giving you the gift then you have no obligation to return the money. Whether it was his money or not, he obviously had control over the funds enough to give you the money. You do not have any obligation or liability to look into your uncle's financial matters to determine if they money that is given to you was his. You can legally accept the fact that when he gives you a gift that he as sufficient legal authority over the money to give it to you.
If your aunt wants the money back she can go after your Uncle for the money. However she has no legal claim against you for the money unless she can prove he was mentally incompetent or you unduly influenced him.
Further, you have no obligation to return any call from the attorney. if you choose to return the call then I suspect they are going to try and scare you into returning the money. If you agree to return the money then you could find yourself obligated through your agreement. Otherwise one would not be legally obligated here.
I cannot provide you with legal advise. I can provide you with information about the law related to your question. My answer, and any information that you can find online, should not take the place of having a detailed consultation with a lawyer in your area to advise you regarding your specific issues.
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